The present invention relates generally to telecommunication systems, more particularly to the co-ordination of uplink scrambling codes between different radio network controllers (RNC) and the assignment of such codes to radio connections of user equipment in a code division multiple access (CDMA) communication system. The invention also relates to methods for creating a starting vector used to generate said scrambling codes.
There are several approaches to methods for sharing a common communication medium amongst a plurality of users. These approaches base on the principle to provide each single user with a communication connection having a certain characteristic that makes a user distinguishable from other users sharing simultaneously the same communication medium. When using frequency division multiple access (FDMA), e.g., each user can be distinguished by a distinct frequency band that is permanently assigned to that user for sending and receiving. The users are able to send whenever they want; however, only within a narrow carrier frequency band. When using time division multiple access (TDMA), each user can be distinguished by distinct time slots that are periodically assigned to that user for sending and receiving. The users are able to send and receive using the entire available bandwidth; however, only at certain short periods of time.
In contrast to that, in a code division multiple access (CDMA) communication system a distinction of different users of the shared communication medium is made by applying a plurality of code sequences to the user signals. Thus, user communication signals from a plurality of user equipment (UE) can be transmitted at the same time and using the entire available carrier frequency bandwidth. These systems intend to achieve a band spreading with sufficient bandwidth expansion in order to give so large suppression of interference from radio connections marked with another code that enough isolation is obtained between simultaneous connections and, thus, to permit multiple access.
However, said code sequences must fulfil certain conditions: For identification of a specific user in a communication system, a large number of code sequences is necessary in order to be able to assign each user a unique code sequence. Suitable code sequences must on the one hand comprise good autocorrelation properties for a reliable initial synchronisation and separation of multipath components and on the other hand a sufficiently low cross-correlation in order to suppress interference in a multiple user system.
One common type of code sequences applied for the uplink radio connection between a user equipment (UE) and a radio base station (RBS) are so called pseudo-noise (pn) sequences, which are generated by help of feedback shift registers. Examples for suitable sequences are maximum-length sequences, Gold- or Gold-like codes, or Kasami-codes.
In a direct-sequence (DS) spread spectrum system, the output signal from a normal transmitter is modulated with a local oscillator comprising an uplink scrambling code sequence. In the receiver, signals from all the communication of various users are received and a specific one is selected by assigning the specific code sequence for that user to a correlator to extract the signal energy intended for that receiver, thereby despreading the received CDMA transmission. By that, the desired signal being marked with a certain code sequence can be detected even against a background of strong interference. All other uncorrelated user transmissions appear as noise.
From what has been said above, it becomes apparent that in a proper working system it is important to provide unique code sequences to each user equipment having a communication connection within a certain area in order to be able to detect said user communication and to prohibit crosstalk between different communication connections. In a known solution, which is applied, inter alia, in IS-95, a standardised identity of a mobile station, e.g. its serial number, is used to create an uplink scrambling code that is unique for that particular mobile station. An identity that is not sufficiently long could be extended, e.g., by padding zeros.
It is an object of this invention to provide a method for co-ordination of uplink scrambling codes that are assigned to radio connections between a user equipment (UE) and its serving radio base station (RBS) in a CDMA-based communication system.
It is another object of the present invention to provide an efficient and secure method for assignment of unique uplink scrambling codes.
Yet another object of the present invention is to provide a decentralised but still co-ordinated and network controlled assignment method.
Yet another object of the present invention is to provide an alternative assignment method without need for deallocation of an assigned code after the corresponding connection has been terminated.
Still another object of the present invention is a method to create starting vectors that can be used for the generation of uplink scrambling codes.
Briefly, these and other objects of the present invention are accomplished by dedicating a unique subset of scrambling codes to each radio network controller (RNC) in a network. In particular, each RNC is assigned an offset vector representing a first vector of a continuous succession of initial starting vectors. A unique subset of initial starting vectors is formed by concatenating said offset vector and the binary representation of a determining value within the size range of the dedicated code subset. An idle starting vector is selected either by using a one-dimensional array containing a status bit for each scrambling code and using the array index as determining value or, alternatively, by using a cyclical or random assignment method. A code generating unit can, by help of a selected initial starting vector, generate the scrambling code from an appropriate code family.
As a first advantage, the present invention comprises a method for co-ordination and assignment of unique uplink scrambling codes to the radio connections between an user equipment (UE) and a radio base station (RBS) in a CDMA-based communication system.
As another advantage of the present invention, the decentralised assignment method within the radio network improves system maintenance and security in case of unexpected disturbances.
Yet another advantage of the present invention is an increased flexibility and, at the same time, a reduced complexity in the system components achieved by distributing the functionality for generating and managing of uplink scrambling codes.
Yet another advantage of the present invention is a decreased signalling traffic due to the decentralised assignment method within the radio network.
Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.